Cochlear implants (CIs) are surgically-implanted prosthetic devices that can provide the profoundly deaf with sensations of sound. In normal hearing, incoming sound is frequency-analyzed by the inner ear's cochlea. High frequency sounds are picked out near the base of the cochlea while low frequency sounds resonate within the cochlea's apical region. Nerve cells disposed within those different regions detect the vibrations and then transmit corresponding nerve impulses to the brain where the impulses are perceived as sound.
In an attempt to replicate this place-frequency map in electric hearing, doctors implant electrodes at specific insertion depths in the cochlea. The electrodes are then activated by incoming sound energy, where sound waves of a particular frequency cause the activation of one or more electrodes positioned at a particular location or depth within the cochlea. When an electrode is activated, the electrode stimulates the nearby tissue with a number of electric current pulses.
The electric pulses are detected by surviving auditory nerve fibers near that region. The electric pulses are typically generated at a constant rate, resulting in a sensation of sound. The louder an incoming sound, the more current is delivered with each of these pulses. Different frequencies of incoming sound waves are reflected in the cochlear implant by stimulating electrodes at different depths within the inner ear.
While cochlear implants improve the quality of life for hundreds of thousands of people worldwide, electric hearing still lacks the resolving ability found in normal hearing. For example, users of cochlear implants have difficulty with pitch detection and sound source localization.
Electric hearing does not operate in the same manner as normal hearing. In normal hearing, the nerve firings that result from cochlear excitation by an incoming sound wave are timed in accordance with the shape of waveform of the incoming wave, a phenomenon called phase locking. As a result, the nerve firings are not strictly periodic because the shape of the incoming waveform is somewhat random. In contrast, the constant rates of conventional cochlear implant stimulation are strictly periodic and can lead to a process called adaptation, whereby the nervous system ignores the repetitive periodic signal. As a result, at least some of the stimulation signals are ignored by the nervous system, reducing information transfer. Exacerbating this condition, the electric pulses of cochlear implants induce “super” phase locking, removing the natural randomness of the timing of nerve firings. It is thought that these deleterious effects are partially responsible for the reduced ability of bilaterally-implanted cochlear implant listeners' to take advantage of binaural cues in comparison with listeners having two normally-functioning ears.
Listening in noisy environments is notoriously difficult for cochlear implant users. For example, many users of cochlear implants report substantial deterioration of speech perception in noisy environments. In fact, noisy or complex sound environments can be unpleasant for users of cochlear implants. These difficulties stem from the inability of cochlear implant users to segregate target sounds and background maskers. Contemporary cochlear implant processing does not preserve many of the physical attributes of sounds that make them unique, such as temporal fine structure.
Therefore, it would be desirable to have a system and method for assisting with sound reception and perception that extracts and presents temporal fine structure from acoustic signals, a feat not achieved by traditional hearing assistance devices, such as traditional cochlear implants.